This invention relates to improvements in heart pacemakers and, more particularly, to improvements in pacemakers of the type which are externally programmable so as to vary selected operating parameters and, more particularly, to the circuit means for generating signals in the implanted pacemaker to indicate the digital states controlling the operating parameters.
As set forth in the above referenced application, the prior art shows a variety of techniques adapted to pacemakers for externally controlling selected operating parameters after the pacemaker has been implanted in a patient. Parameters of interest may include the stimulation pulse amplitude, width and rate, as well as the refractory period the sensitivity to naturally occuring heart pulses, and the mode of operation.
U.S. Pat. No. 4,049,004, assigned to the assignee of the present invention, discloses a heart pacer having variable operating parameters which are controlled by digital circuitry responsive to externally applied magnetic pulses. Said U.S. Pat. No. 4,049,004 is incorporated herein by reference.
Referenced U.S. Pat. No. 4,190,555 provides a modification of the pacemaker of the referenced patent so as to provide pulse position modulation of the generated pacer pulses. In that arrangement, a delay is added between pulses when a particular logic state appears, and no delay is added if the compliment of that logic state appears. This means that the parameters can be determined externally by observing the successive periods between stimulation pulses when the pacer is placed in a magnetic mode.